Van Mai Cao-Lormeau, a senior scientist in virology at the Institut Louis Malardé in Papeete, Tahiti, is leading much of the small research center's work to understand a recent Zika outbreak in French Polynesia.Karim Mahdjouba for STAT

PAPEETE, Tahiti — As scientists around the world race to understand the threat posed by the Zika virus, they are turning to this remote and relaxed Pacific island, which recently experienced an outbreak.

Zika roared through here between October 2013 and May 2014, sending more than 30,000 residents — 10 percent of the population — to doctors and hospitals for treatment. The Polynesian archipelago was the first place of any size where the virus had emerged, and a team of top scientists at a small research institute watched the outbreak unfold in real time.

The Institut Louis Malardé — whose open-air central courtyard was draped this week by a bright red bougainvillea in full flower — tracked Zika’s spread, aided by machinery that rapidly sequences the DNA of viruses and patient records from the main hospital and clinics. Now its five PhD scientists and physicians are hurriedly drafting journal articles to report their latest findings.

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“We are trying to document as much as possible what happened in these people,” Van Mai Cao-Lormeau, a senior scientist in virology at the institute, told STAT Monday. “This will be very useful in the kind of investigation now being conducted in Latin America.”

Scientists from the US Centers for Disease Control and Prevention, France’s Institut Pasteur, and University of Oxford in the UK have communicated with the Polynesian team in recent weeks to learn all they can about Zika. The mosquito-borne virus likely spread from here to Brazil in 2014 and is now circulating in two dozen countries in South and Central America and the Caribbean.

In a paper slated for publication soon, the Polynesian researchers will examine the intriguing possibility that the dozens of Zika patients here who came down with Guillain-Barré syndrome, an automimmune condition that can cause temporary paralysis, had previously been infected with dengue, a virus in the same family as Zika.

An immune system that recently fought off dengue, which is endemic in the tropics, might overreact to Zika, triggering Guillain-Barré, the researchers speculate.

Cao-Lormeau (right) and Dr. Tu-Xuan Nhan, assistant director of the Diagnosis Laboratory, at the Institut Louis Malardé, where they are studying a recent Zika virus outbreak.Karim Mahdjouba for STAT

They declined to detail their evidence ahead of publication, but at least some of the GBS patients treated at a rehabilitation clinic in the hills outside Papeete were infected with dengue shortly before they came down with Zika, Stéphane Milon, director of care at the Te Tiare clinic, said Monday.

The possible link between Zika and Guillain-Barré was first established here, with 42 people diagnosed during the outbreak, after just a handful the entire year before.

While infectious diseases like dengue and chikungunya are known to cause GBS too, the number of cases linked to those viruses is typically much lower than was seen in French Polynesia.

Since Zika arrived in Latin America, countries such as Brazil, Colombia, Venezuela, and El Salvador have similarly reported upticks in Guillain-Barré cases. Researchers there are likewise studying whether Zika alone can increase the risk of GBS, or if other factors, such as a previous dengue infection, are needed.

The disease, known for triggering temporary paralysis, has been linked to Zika virus infections.Hyacinth Empinado/STAT

While a connection to Guillain-Barré was immediately obvious in Polynesia, because the uptick in patients coincided with the start of the outbreak, the rise in babies with the telltale small heads of microcephaly was noticed only in recent weeks, when researchers at a Papeete hospital went back to look at the medical records of women who were pregnant during the outbreak.

They found that 12 women had been told their fetuses had such neurological defects, said Cao-Lormeau, who is leading much of the research. Nine of the 12 women had abortions, and the other three gave birth to babies with significant problems, she said.

In previous years, at most only two Polynesian babies a year had such birth defects, which cause intellectual disability and lifelong health problems and have no treatment or cure.

Isolated island chains such as French Polynesia are a perfect laboratory for studying infectious diseases, in part because it is clear when outbreaks begin, said Adam Kucharski, a lecturer in infectious disease epidemiology at the London School of Hygiene and Tropical Medicine.

In Brazil and other parts of the Americas where Zika is now raging, there are other mosquito-borne viruses, making it difficult to determine which disease is causing which effects.

Tahiti experienced an outbreak of chikungunya, another mosquito-borne virus, last year, and a small number of dengue cases crop up every few months. But it’s easier, Kucharski said, to pin the blame on one disease when scientists have a clear idea when the disease arrived and how it spread.

Cao-Lormeau said it was very clear that the outbreak in French Polynesia began around October 2013, when a small group of people on Tahiti came down with the telltale itchy rash, blood-red eyes, and fever. No one in that first group could have brought the virus, because they hadn’t traveled off the island in preceding weeks.

But Cao-Lormeau, who came down with Zika herself, and her colleagues have been tracking the viruses infecting Polynesian residents since 2012, so they are certain that there was no Zika before late 2013.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions.Alex Hogan/STAT

Rumors around Tahiti suggest that the virus arrived with an African beach soccer team that came to play in an international tournament around the same time. Cao-Lormeau’s genetic analysis shows that cannot be the case, however; the Zika in Polynesia came from Asia, and is very similar to a strain that sickened about 7,000 people in the Yap Islands of Micronesia in 2007. That was the first known outbreak of the virus, which had been discovered and named in Uganda in 1947. Only 14 individual cases had been reported previously.

Scientists don’t know whether the virus became more dangerous as it crossed the South Pacific, or whether its possible connection to Guillain-Barré and microcephaly went unnoticed in the Yap outbreak because it was relatively small and in an area that lacked close scientific surveillance.

About 20 percent of those infected with Zika show any symptoms, scientists estimate, so they assume that well over half the population of French Polynesia caught the mosquito-borne virus before it died out.

“We are trying to document as much as possible what happened” during the recent Zika outbreak, says Cao-Lormeau.Karim Mahdjouba for STAT

Genetically tracking the virus is trickier than it was, say, with West Africa’s Ebola epidemic, Cao-Lormeau said. There, epidemiologists and genetic researchers were able to track the evolution of the Ebola virus as it migrated from a 2-year-old child in Guinea to two midwives in nearby Sierra Leone and then across the border into Liberia.

Because Zika is transmitted by mosquitoes, the virus is evolving in the bugs, not people. Also, people clear the Zika virus very quickly from their bloodstream, Cao-Lormeau said, making it difficult to obtain samples.

The breed of mosquito that commonly transmits Zika, Aedes aegypti, is present in French Polynesia, Cao-Lormeau said, alongside an indigenous mosquito that was also able to spread the virus once it arrived.

Zika arrived in French Polynesia via Tahiti, its most populous island, and then spread to the other 66 inhabited islands of the archipelago, she said, although there are different types of habitats — from the lush forested hillsides rising above urban Papeete to rural islands composed of coral and volcanic rock.

As with most island outbreaks, Zika left French Polynesia as quickly as it came, once everyone who was susceptible became infected and the virus ran out of new victims. By early May 2014, there were no more reports of Zika in Tahiti, though the virus continued its march eastward.